Patent Document 1 describes providing a selective reduction type NOx catalyst (also referred to hereafter as an SCR catalyst) that selectively reduces NOx contained in exhaust gas by adsorbing ammonia (NH3) in an exhaust passage of an internal combustion engine, and calculating an amount of ammonia consumed by the SCR catalyst on the basis of an amount of NOx discharged from the internal combustion engine and a NOx purification ratio of the SCR catalyst. Further, Patent Document 1 describes calculating an amount of ammonia adsorbed to the SCR catalyst in accordance with the amount of consumed ammonia and an amount of added reducing agent, and adjusting the amount of added reducing agent in accordance with the adsorption amount.
Incidentally, the ammonia adsorbed to the SCR catalyst may decrease even when NOx is not purified thereby. For example, when a temperature of the SCR catalyst increases, ammonia is desorbed from the SCR catalyst, and therefore the amount of ammonia adsorbed to the SCR catalyst decreases. Hence, an estimated value of the amount of ammonia adsorbed to the SCR catalyst may be larger than an actual value. When the amount of added reducing agent is adjusted in accordance with the estimated value, therefore, the amount of added reducing agent may be inappropriate.